Modeling means that a model is created from the object under design for describing the object to be manufactured. The development of data processing systems and computers has transformed modeling into a computerized process, where a product model is created from the object. For example, the product model of a building is an entity consisting of the product data on the life span of the building and building process. The product model of a building describes the product data of the building outlined in accordance with the product data model. The building product model may be stored as a database of a computer application or as a file suitable for data transfer. Computer applications describe the real-world building elements of the building using building element entities that are processed by the applications.
The modeling by means of computer applications involves two different approaches to modeling: bottom-up and top-down. The bottom-up approach is based on a principle of forming a piece from details; each edge of the piece is drawn in the program and the piece is formed as a sum of all details. The formation of pieces is facilitated by a library where all piece forms included in the design have been stored. A ready-made piece form can be selected from the library and its different parts may be given parameters, whereby several different pieces can be obtained from one piece form. If the number or order of edges changes, a new piece form is stored in the library. The top-down approach is based on the principle of providing a piece with details; a piece is created first and then provided with details. The top-down modeling has been facilitated by dividing an entity into smaller entities, such as separate building elements, joints and details programmed into the modeling program. The most typically used building elements can be created by combining smaller programmed entities known as program snippets. Objects are modeled by parametrizing, i.e. by giving start and end points as well as definitions defining various properties of the piece, such as location, material or type of cross section. Pieces can also be parametrized according to the environment. The top-down approach differs from the bottom-up approach also in that the piece form is not stored in the library but the outcome is only a set of the piece's parameters (set of definitions) in the design. There is no piece form library similar to the one employed in the bottom-up approach. Top-down modeling is described in greater detail in the applicant's U.S. patent application Ser. No. 11/165,043, where the parametric modeling described represents top-down modeling.
A problem associated with top-down modeling is, in fact, caused by the lack of a piece form library, for which reason there is no link between identical and similar pieces. When a drawing, for example, is made of a piece, the user sets the desired view angles and measures for the drawing and the same drawing, more precisely the drawing template provided by the drawing, could be used for pieces of the similar type. In the bottom-up approach, this is easy in the case of a stored piece form because a drawing can be linked as a drawing template with a library element. In that case, the same drawing template can be used when a drawing is made of a piece created using the same piece form. In the top-down approach, this is not feasible because there is no piece form library, and thus the user has to make the drawing of each piece separately. The same problem comes up both in the top-down and in the bottom-up approach when a drawing is made of an object that is an entity consisting of several pieces or when detailed drawings are made of similar parts of a piece.